User blog comment:DarkSlayer 3/Theories/@comment-7738747-20130722153254/@comment-775152-20130724021356

Yeah, it's not very likely that Vergil would have ... um... with Alice out of actual love. But, he is a man, and men have... tendencies... If that's still not believable, we can consider that Arkham is a master manipulator, and Vergil is (argueably) more easily manipulated than Dante. There may have been some casting of "Suggestion" spells involved.

Or, I could be totally wrong. It's all just speculation!